Ferroelectric materials have been used to form electrical components such as non-volatile memories, capacitors, and optical guided wave devices. For example, a ferroelectric gate transistor consists of a ferroelectric insulator disposed over a semiconductor substrate and a gate electrode disposed over the ferroelectric material. The gate material and the ferroelectric material are etched to form a gate structure. By polarizing the ferroelectric material so that the electric charge is toward or away from the semiconductor surface, the ferroelectric material is programmed to remain in that state, and the semiconductor is induced to be in an inversion or accumulation mode under the ferroelectric material. This in turn makes the device conductive or insulating in the lateral direction, depending on the channel type of the semiconductor.
The hysteresis in the polarization of the ferroelectric materials make it possible to use such materials in nonvolatile memory devices. By using the ferroelectric material in the gate insulator stack of a field effect transistor (FET), either alone or in conjunction with other dielectric materials, the threshold voltage of the FET is determined by the polarization of the ferroelectric material layer. In the prior art, the ferroelectric material layer is uniform and, hence, the FET has two threshold voltages, determined by the positive and negative remnant polarization of the ferroelectric material layer to provide a single state (on/off) memory device.
Accordingly, it would be highly desirable to provide multi-state ferroelectric gate field effect transistor memory devices.
It is a purpose of the present invention to provide ferroelectric gate field effect transistor memory devices having more than one (on/off) states.
It is another purpose of the present invention to provide ferroelectric gate field effect transistor memory devices having more than two threshold states.